This invention relates to pagers, and more particularly to a pager which selectively uses a built-in primary battery as the power source of the receiver, or an external power source.
In a radio-call service using a pager, because of the provision of a FLEX-TD system the amount of information has increased, and the service has become diversified.
An example thereof is an information service. A pager is fundamentally used for a private calling service; however, the information service etc. are not limited thereto or thereby. The information service is such that the radio-call service provider and the content provider cooperate with each other, and the property of simultaneous transmission of the pager is utilized to transmit various news to a unspecified users. In order to transmit news to a number of persons, there is available a pager with an external output terminal which has a function of displaying on an electrical display board the news which are received by the pager.
FIG. 6 shows an example of the arrangement of a power source circuit for a conventional pager with an external output terminal. In FIG. 6, reference numeral 10 designates an air battery, which is employed as a power source built in the pager. Reference numeral 102 designates a DC/DC converter which forms a system voltage for the pager. Reference numeral 103 designates an external terminal, which is used for connection of external equipment. The external terminal 103 includes an input/output terminal and a power source terminal which is connected to an external power source. Reference numeral 104 designates a voltage conversion circuit, which functions to decrease the voltage obtained through the power source terminal of the external terminal 103 to a battery voltage. The voltage conversion circuit 104 comprises a regulator (REG) 104a for forming a constant voltage, and a reverse current preventing diode 104b. 
The pager uses as its power source either the air battery 101 built therein or the external power source connected to the external terminal 103.
However, it is impossible for the pager to use both of the air battery and the external power source at the same time. This is because, if the air battery and the external power source are used at the same time, the voltage provided by the voltage conversion circuit 104 is applied to the primary battery, namely, the air battery 101, to charge the primary battery, which may break the latter. In order to prevent the primary battery from being charged, a method may be employed in which a reverse current preventing diode is coupled to the primary battery. However, the method is not practical because it results in a voltage loss. Accordingly, in the case where the above-described conventional pager uses the air battery, it is necessary to electrically disconnect the voltage conversion circuit 104 from the external terminal 103, or to insert a resistor to prevent the use of the current of the external power source. In addition, in the case where the external power source is used, it is necessary to structurally inhibit the insertion of the air battery.
As is apparent from the above description, for the conventional pager there is no means for avoiding the problem (difficulty) that, in the case where the pager uses the primary battery and the external power source at the same time, the primary battery is charged. Hence, the user must pay attention to the operation of the pager, and the latter is unavoidably intricate in structure. Even if it is tried to provide common parts or common structures, it is necessary to prepare special parts for the power source section. This means that the manufacture of the pager is low in manufacture control efficiency.
Accordingly, an object of the invention is to eliminate the above-described difficulties accompanying a conventional pager.
More specifically, an object of the invention is to provide a pager in which, even when the primary battery and the external power source are used at the same time, the primary battery is not charged, so that it is high in safety, and the occurrence of troubles is prevented during use.
Another object of the invention is to provide a pager which is high in manufacture efficiency with the parts and structure being simplified irrespective of the purposes of use thereof.
The foregoing objects of the invention have been achieved by the provision of a pager comprising first power source means in which a primary battery is employed as a power source, and second power source means in which an external power source supplied from outside is employed as a power source, which comprises: charge preventing means which, when the first and second power source means are used at the same time, prevents the primary battery in the first power source means from being charged by the external power source in the second power source means.
With the pager, the difficulty is eliminated that, when the primary battery and the external power source are used at the same time, erroneously the primary battery is charged.
In the pager, the first power source has a first positive terminal and a first negative terminal which are connected both terminals of the primary battery, and a battery detecting terminal which, when the primary battery is mounted on the pager, is electrically connected to the first negative terminal, and when the primary battery is not mounted on the pager, is electrically disconnected from the first negative terminal, and the second power source has an external terminal which has at least a second positive terminal and a second negative terminal which are connected to the external power source, the second positive terminal being connected through a resistor to the battery detecting terminal.
With the pager, in the case where the primary battery and the external power source are used at the same time, the second positive terminal is connected through the resistor to the battery detecting terminal and the first negative terminal. This feature prevents the current of the external power source from flowing to the power source path of the primary battery.
The above-described pager comprises external power source detecting means for detecting the fact that the power of the power source is applied to the second power source means.
With the pager, the fact is detected that the power source is connected, and its power is applied to the second power source means.
Furthermore, the pager comprises simultaneous-use detecting means for detecting the fact that the first and second power source means are used at the same time.
With the pager, the fact is detected that the power source of the built-in primary battery and the external power source connected to the external terminal are used at the same time.
In addition, the pager comprises alarming means which, when the simultaneous-use detecting means detects the fact that the first and second power source means are used at the same time, outputs a warning signal for a user.
With the pager, for instance when the built-in primary power source and the external power source are used at the same time, the user can detect the state of use of the power source with ease.